This invention relates generally to video inspection and more particularly to a zoomable, focus adjustable and aperture variable optical system for video inspection devices.
Video inspection devices, such as video endoscopes, can be used to inspect target objects to identify and analyze flaws and defects in the objects both during and after an inspection. Often times, inspections are performed in small environments such as inside aircraft engines and small pipes. Typically, these environments are poorly illuminated with the only source of light being the endoscope's own light source (e.g., a fiber optic bundle that transmits light from a proximally located lamp, LED or laser, or a distally located lamp or LED). In order for video inspection devices to be effective in maneuvering in these environments, they must be of small diameter, typically less than 10 mm, and exhibit a small distal rigid length, typically less than 20 mm, in order to navigate through locations containing small bending radii. In addition, video inspection devices require versatility in providing wide fields of view, typically up to 120 degrees or greater, in order to provide a more complete view of a target object, while at the same time providing a large depth of field for maintaining a clear image when the inspection device is placed in close proximity to an object. Detailed inspection of objects often requires the ability to observe them from very close distances, typically less than 5 mm. At close distances the illumination light is typically sufficient to produce a bright, high quality image, but a sufficient depth of field is required for viewing 3D objects. Other times, general device navigation and inspection applications require inspectors to view objects from greater distances. When the inspection device is located further away from a given inspection target maintaining sufficient image brightness may be problematic. As such, video inspection devices require an aperture capable of compensating for variable lighting requirements and depths of field as the device is moved closer to or farther from a given inspection target.
Video inspection systems today are typically comprised of a primary optical system exhibiting a fixed, far focused, small field of view on the order of 40 to 50 degrees. The primary optical system provides a large depth of field, highest contrast and brightest image possible for a given endoscope. This arrangement allows for general navigation and distant general inspection of an object of interest, in addition to limited magnification change achievable by pushing the lens system closer to the inspection target within its depth of field. To provide increased magnification at closer focus distances and/or a wider field of view, typically up to 120 degrees, various optical systems can be installed on the forward end of the inspection probe to alter the optical characteristics of the probe and provide a desired field of view and focus distance. These optical systems are often referred to as tips and can be removable. The drawback of this arrangement is that each tip provides a discrete field of view and focus distance change. In addition, the inspector operating the video inspection device is required to pull the probe back out from the inspection site, change the tip, and reinsert the probe if the inspector wishes to change the level of magnification. This can be a time consuming process, and can result in the inspector's inability to relocate the specific site of interest. Alternatively, there are also manually adjustable tips for which the focus can be set prior to inspection. However, such tips can be difficult for the inspector to use since the inspector must know at what distance to focus the tip before using it, and then subsequently be able to accurately set that focus.
Video inspection devices today generally lack an auto-focus mechanism and contain optical systems that are designed to have a fixed focal length with a large depth of field that is sufficient to produce adequate image quality over the typical range of inspection distances. Having this fixed focal length results in all but a limited range of probe focus distances not having the best focus, image quality, contrast and resolution possible. In addition, in order to achieve a large depth of field, a small aperture size is commonly utilized, decreasing image brightness and achievable image contrast and resolution. Typically, the aperture is of a fixed size and position, designed both to facilitate a desired depth of field given a particular lens system diameter, and to minimize distortion and other optical aberrations associated with a given lens arrangement, resulting in a fixed F Number. This inherently results in conflicting goals of achieving good image quality within an acceptable depth of field at close inspection distances while providing a clear, bright image at farther inspection distances.
The design of a video inspection system able to perform optical zooming, focus adjustment and aperture variation is constrained, in part, because of space limitations associated with narrow diameter inspection probes, and the inability to place an optical system capable of varying its focal length within the limited space of the probe. Some devices attempt to provide optical zoom and focus adjustment capabilities using costly, complex optical arrangements consisting of numerous lenses that are difficult to fit within an endoscope. Although several existing video inspection devices offer a zooming capability, the zoom feature is typically that of a digital zoom, a method limited by the resolution provided by the charged coupled device (CCD) or imager. In digital zoom systems, as the zoom level is increased, the resolution and image quality decreases.
The physical conditions of the typical video inspection environment in which video inspection devices operate require that inspection devices be versatile and rugged. In situations in which the probe is used to inspect small diameter pipes, the ability to view the environment from a side-facing surface of the probe rather than the forward-facing surface would be beneficial. Because of these space limitations, the distal end of the video inspection device must be of a fixed rigid length. In addition, the ability to operate the video inspection device in environments up to 80 degrees Celsius, such as a hot engine or turbine, is sometimes necessary and cost effective, as opposed to first waiting for the engine or turbine to cool down before performing the inspection. In situations in which the video inspection device is exposed to liquid environments, such as water, excellent sealing of the device to prevent the liquid from entering the probe is necessitated. Finally, because the typical video inspection environment can be an industrial setting that subjects the probe to potential dropping or being struck by other objects, video inspection devices should be mechanically strong enough to endure harsh environments and accidental mishandling.
One potential method of providing a focus and zoom capability is the use of a liquid lens. In a liquid lens, the optical properties of the lens are altered by the electrostatic pressure applied to a transparent, non-permeable membrane that separates two liquid materials, typically water and oil, when a voltage is applied across those liquids. However, the liquid lens solution has limited practical applicability in a video inspection device as the technology today requires a large diameter lens fixture, has a narrow working temperature range typically up to 60 degrees Celsius, and requires a high drive voltage. Similarly, other conventional zoom and focus systems employed in cameras outside of the video inspection device field are limited in their application to the video inspection field by their size and inability to operate in the extreme environments encountered in the inspection field. It would be advantageous to provide a video inspection device that provides a zoomable, focus adjustable and aperture variable optical system suitable for use in an endoscopic inspection environment.